1. Field of the Invention
The present invention pertains to a system and method for providing a pressure support therapy to a patient, and, in particular, to a pressure support system and method of operating such a system that minimizes the operating speed of the pressure generating component of the system.
2. Description of the Related Art
It is well known to treat a breathing disorder, such as obstructive sleep apnea (OSA), with a pressure support device, such as a continuous positive airway pressure (CPAP) device. A CPAP device delivers a flow of fluid to the airway of the patient throughout the patient's breathing cycle in order to “splint” the airway, thereby preventing its collapse during sleep. The term “fluid” as used herein refers to any gas, including a gas mixture or a gas with particles, such as an aerosol medication, suspended therein. Most commonly, the fluid delivered to a patient by a pressure support system is pressured air. An example of such a CPAP device is the REMstar® and Solo® family of CPAP devices manufactured by Respironics, Inc. of Pittsburgh, Pa.
It is also known to provide a bi-level positive pressure therapy in which the pressure of fluid delivered to the patient's airway varies or is synchronized with the patient's breathing cycle to maximize the therapeutic effect and comfort to the patient. An example of a pressure support device that provides “bi-level” pressure support, in which a lower pressure is delivered to that patient during the patient's expiratory phase than during the inspiratory phase, is the BiPAP® family of devices manufactured and distributed by Respironics, Inc. of Pittsburgh, Pa. Such a bi-level mode of pressure support is taught, for example, in U.S. Pat. No. 5,148,802 to Sanders et al., U.S. Pat. No. 5,313,937 to Zdrojkowski et al., U.S. Pat. No. 5,433,193 to Sanders et al., U.S. Pat. No. 5,632,269 to Zdrojkowski et al., U.S. Pat. No. 5,803,065 to Zdrojkowski et al., U.S. Pat. No. 6,029,664 to Zdrojkowski et al., U.S. Pat. No. 6,305,374 to Zdrojkowski et al., and U.S. Pat. No. 6,539,940 to Zdrojkowski et al., the contents of each of which are incorporated by reference into the present invention.
It is further known to provide an auto-titration positive pressure therapy in which the pressure provided to the patient changes based on the detected conditions of the patient, such as whether the patient is snoring or experiencing an apnea, hypstokes opnea, cheynes-respiration, or upper airway resistance. An exemplary auto-titration pressure support mode is taught, for example, in U.S. Pat. Nos. 5,203,343; 5,458,137 and 6,085,747 all to Axe et al., the contents of which are incorporated herein by reference. An example of a device that adjusts the pressure delivered to the patient based on whether or not the patient is snoring is the Virtuoso® CPAP family of devices manufactured and distributed by Respironics, Inc. An example of a pressure support device that actively tests the patient's airway to determine whether obstruction, complete or partial, could occur and adjusts the pressure output to avoid this result is the Tranquility® Auto CPAP device and REMStar Auto CPAP device, also manufactured and distributed by Respironics, Inc. This auto-titration pressure support mode is taught in U.S. Pat. Nos. 5,645,053 and 6,286,508 6,550,478 all to Remmers et al., the content of which is also incorporated herein by reference.
Other pressure support systems that offer other modes of providing positive pressure to the patient are also known. For example, a proportional assist ventilation (PAV®) mode of pressure support provides a positive pressure therapy in which the pressure of gas delivered to the patient varies with the patient's breathing effort to increase the comfort to the patient. U.S. Pat. Nos. 5,044,362 and 5,107,830 both to Younes, the contents of which are incorporated herein by reference, teach a pressure support device capable of operating in a PAV mode. Proportional positive airway pressure (PPAP) devices deliver breathing gas to the patient based on the flow generated by the patient. U.S. Pat. Nos. 5,535,738; 5,794,615; and 6,105,575 all to Estes et al., the contents of which are incorporated herein by reference, teach a pressure support device capable of operating in a PPAP mode. In the PAV and PPAP pressure support systems, the percent of assistance provided by the unit is at least one of the operating features of the pressure support device that is set after the device has been prescribed for use by a patient.
A typical conventional pressure support system 10 is shown in FIG. 1. Such a system includes a pressure generator 12 that receives a supply of gas from a gas source, such as ambient atmosphere, as indicated by arrow A, and creates a flow of breathing gas, as indicated by arrows B, having a pressure greater than the ambient atmospheric pressure. Pressure generator 12 typically includes a motor driving a blower, which is an impeller within a housing, for placing the gas from the gas source under pressure relative to ambient atmosphere. A valve 13 downstream of pressure generator 12 bleeds off excess pressure/flow from the patient circuit, as indicated by arrow C, by communicating a portion of the gas at the output of pressure generator 12 to ambient atmosphere. Other conventional exhaust valves divert all or some of the exhaust gas back to the inlet of the pressure generator.
A patient circuit 14, which is typically a flexible conduit, delivers the elevated pressure breathing gas to the airway of a patient 15. Typically, the patient circuit is a single limb conduit or lumen having one end coupled to the pressure generator and a patient interface device 16 coupled to the other end. Patient interface device 16 connects patient circuit 14 with the airway of patient 15 so that the elevated pressure gas flow is delivered to the patient's airway. Examples of patient interface devices include a nasal mask, nasal and oral mask, full face mask, nasal cannula, oral mouthpiece, tracheal tube, endotracheal tube, or hood. A single limb patient circuit shown in FIG. 1 includes an exhalation port 18, also referred to as an exhalation vent, exhaust port, or exhaust vent, to allow gas, such as expired gas from the patient, to exhaust to atmosphere, as indicated by arrow C. Generally, exhaust vent 18 is located in patient circuit 14 near patient interface device 16 or in the patient interface device itself.
More sophisticated pressure support devices include a flow sensor 20, pressure sensor 22, or both to monitor the flow and/or pressure of gas in patient circuit 14. The flow information can be used to determine the volume of gas passing through patient circuit 14. The information from flow sensor 20 and/or pressure sensor 22 is provided to a controller 24, which uses it for any conventional purpose, for example, to control the pressure or flow of gas provided to the patient, monitor the condition of the patient, monitor the usage of the pressure support device (patient compliance), or any combination thereof. An input/output device 26 communicates with controller 24 to provide data, commands, and other information between a user or other entity and the controller.
In a conventional CPAP system, the pressure output by the blower varies with the rate of flow in the patient circuit, assuming that the blower operates at a constant speed. For example, at a certain operating speed, the pressure in the patient circuit or patient interface decreases as the flow of gas in the patient circuit or at the patient interface increases. This occurs, for example, as the patient breathes into the patient circuit. For this reason, a conventional CPAP pressure support system typically uses valve 13 to regulate the pressure delivered to the patient by means of controller 24.
Devices that provide a bi-level positive pressure therapy, an auto-titrating pressure level, PAV, PPAP, or any other mode of pressure support where the pressure delivered to the patient's airway varies, also include a pressure/flow control system to vary the pressure delivered to the patient in accordance with the pressure support mode. A typical pressure/flow control system used in such devices includes the pressure/generator and valve combination shown in FIG. 1.
Pressure support systems that include the pressure/generator and valve combination typically operate the blower at a substantially constant speed that is sufficient to deliver a pressure in excess of the selected pressure to be delivered to the patient. That is, the pressure output by the blower, which is referred to as the “deadhead pressure”, must be high enough above the selected pressure, or range of pressure, to deliver the prescribed pressure to the patient's airway. Moreover, because it is not known at what air density any given pressure support system will be operated, each system is typically preprogrammed during manufacture to operate at an output pressure that is sufficient to account for the lowest air density in which the unit will be operated, such as at a high elevation, a lower barometric pressure, high temperature, or a high humidity.
It can be appreciated that operating the pressure generator to compensate for the worst case scenario requires outputting much more pressure than needed from the pressure generator. Moreover, bleeding off excess pressure or flow from the patient circuit via the exhaust valve is relatively noisy, especially in light of the fact that pressure generator is operating at a high speed, which itself adds to the machine noise.